Field of the Invention
This invention relates to a method to manufacture an optically transparent plastic strip, that could be film or sheet, used to secure information contained on a credit card or identification card. More particularly, an optically variable device, such as a diffractive pattern or hologram, is embossed into a thermoplastic during an extrusion process.
Description of the Related Art
The use of plastic strips with overt and covert security features has become prevalent in the manufacture of secure cards used in applications such as financial transactions and identification cards. Plastic strips with laser engravable additives have added to the functionality and the ability to secure such cards. To further enhance the security of the card, diffractive structures, such as holograms, are added to a secondary film, and then the secondary film is laminated to a plastic overlay. Alternatively, the secondary film is bonded to the plastic overlay by hot stamping. In yet another alternative, secondary diffractive coatings are cast onto a polycarbonate film. To protect the integrity of the diffractive structure, the secondary film is usually covered with a topcoat.
The use of a secondary film increases cost and complexity. The bond between the secondary film and the plastic overlay film creates a possible security risk by forming a potentially separable layer to the overall structure. The necessity of a protective topcoating adds further complexity to the overall construction.
U.S. Pat. No. 5,331,443 discloses a composite film having a hologram embossed into one film layer and coated with a reflective metalized layer. A laser selectively etches unique indicia into the metalized layer. The composite film is bonded to a substrate that may be transparent or opaque.
U.S. Pat. No. 5,815,292 discloses a first transparent film layer that is embossed with a first surface relief diffracting pattern and partially coated with a first metallization layer. The first transparent film layer is bonded to a second transparent film layer having a second surface relief diffracting pattern and coated with a second metallization layer. The film assembly is then bonded to a substrate to be authenticated with an adhesive. The adhesive is stronger than any of the film assembly bonds such that the assembly will be damaged if an attempt is made to remove it from the substrate.
United States Published Patent Application, Publication No. US 2009/0251749 A1, discloses a hologram containing overlay for fusing to a substrate. The overlay has a polycarbonate base, a diffractive pattern is formed on the base and coated with a reflection enhancing layer. The diffractive layer is preferably applied by a casting process, although embossing may be utilized. Embossed holograms are disclosed to be less desirable than cast holograms because the conventional embossed hologram base is a thermo compliant material; thus the image is formed by applying pressure only. As a result, it is subject to distortion in a vigorous lamination or embedding process required for polycarbonate card manufacturing.
Each of U.S. Pat. Nos. 5,331,443 and 5,815,292 and Publication No. US 2009/0251749 A1 is incorporated by reference herein in its entirety.
There remains a need for a method to emboss a diffractive structure directly into a plastic strip, without a need to subsequently apply a protective topcoating, that provides not only advantages in reduced complexity and cost, but also enhances the overall security of the structure as the diffractive image is within the film, and can only be removed or altered by destroying the plastic strip.